1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical regeneration circuit for regenerating a signal having a deteriorated signal to noise ratio.
2. Description of the Related Art
FIG. 1 shows a schematic configuration of a conventional optical transmission system using an optical direct amplifying transmission scheme.
This optical transmission system comprises an optical transmitter 9 for transmitting a signal light; an optical receiver 10 for receiving a signal light; a plurality of optical amplifiers 7 for amplifying the applied signal light and outputting the amplified signal light; a plurality of optical fibers 6 in the forms of rare-earth doped optical fibers, on which excited light is incident, for connecting between optical transmitter 9 and the first one of optical amplifiers 7, between adjacent optical amplifiers 7, and between the last one of optical amplifiers 7 and optical receiver 10.
This optical transmission system has the following problems. Each of optical amplifiers 7, when amplifying a signal light, superimposes a spontaneous emission optical noise generated inside upon the optical signal. Thus, in the optical transmission system having optical amplifiers 7 connected in multiple stages, the spontaneous emission optical noise is accumulated while the signal light is propagated through the optical amplifiers, thereby causing deterioration in the signal to noise ratio. In addition, the signal to noise ratio is also deteriorated by a waveform distortion generated when a signal light is propagated through the plurality of optical fibers 6. Thus, in the conventional optical transmission system, favorable optical signal transmission can not be performed especially in transmission for a long distance.
It is an object of the present invention to provide an optical transmission system capable of favorable transmission over a long distance without deteriorating the signal to noise ratio.
To achieve the above-mentioned object, the optical transmission system according to the present invention, comprises an optical transmitter, an optical receiver, an optical fiber, and an optical amplifier, employs an optical direct amplifying and relaying transmission scheme, is characterized by an optical regeneration circuit on the path of the optical fiber.
The optical regeneration circuit according to the present invention comprises an optical wavelength selector, a light source, an optical coupler, a saturable absorption type optical element, and a wavelength divider.
The optical wavelength selector passes therethrough, from an incident signal light which includes a noise light, only the component with a wavelength of the signal light. The optical coupler means synthesizes the signal light outputted from the optical wavelength selector and the continuous light outputted from the light source to output the synthesized light. The saturable absorption type optical element absorbs almost all the composed signal light received from the optical coupler when the composed signal light has a light intensity lower than threshold value and transmits the composed signal light received from the optical coupler when the composed signal light has a light intensity equal to or greater than the threshold value. The wavelength divider divides only the component with the same wavelength as the wavelength of the continuous light from the composed signal light outputted from the saturable absorption type optical element to output the divided light as a regenerated signal light.
The signal light which includes a noise light is composed with the continuous light at a different wavelength to obtain the composed signal light. This composed signal light passes through the saturable absorption type optical element, thereby transmitting only the composed signal light having a light intensity equal to or greater than the threshold value to remove the noise light. From the composed signal light having the light intensity equal to or greater than the threshold value, a signal with the same wavelength as the wavelength of the continuous light is extracted by the wavelength divider. In this way, the incident signal light is replicated in the continuous light and outputted. Therefore, the signal light having no deterioration of the signal to noise ratio at a wavelength different from the wavelength of the applied signal light can be regenerated from the signal light having the deteriorated signal to noise ratio.
Also, according to an embodiment of the present invention, an optical amplifier is further provided which is interposed between the optical coupler and the saturable absorption type optical element. Thus, a light intensity of a signal light to be applied to the saturable absorption type optical element can be set at an optimal level for the threshold value of the saturable absorption type optical element.
Additionally, another optical regeneration circuit according to the present invention comprises an optical wavelength selector, a light source, a saturable absorption type optical element, and a wavelength divider
When an added value of a light intensity of a continuous light from the light source applied at one terminal and a light intensity of a signal light applied at the other terminal is below a threshold value of a saturable absorption region, the saturable absorption type optical element absorbs almost all the continuous light and the signal light. Conversely, when the added value of the light intensity of the continuous light from the source applied at the one terminal and the light intensity of the signal light applied at the other terminal is below the threshold value, the saturable absorption type optical element allows the continuous light to pass therethrough and provides the light as a regenerated signal light at the other terminal.
The optical divider includes a first terminal connected to the optical wavelength selector, a second terminal connected to the saturable absorption type optical element and an output terminal through which the regenerated signal light is transmitted to the outside. The signal light received from the optical wavelength selector at the first terminal is supplied to the saturable absorption type optical element through the second terminal and the regenerated signal light received from the saturable absorption type optical element at the second terminal is outputted at the output terminal.
In the optical divider, the signal light received from the outside is different from the continuous light outputted from the light source in the traveling direction, so that a wavelength of the signal light received from the outside and a wavelength of the continuous light received from the light source can be made equal to each other.
According to an embodiment of the present invention, the wavelength of the signal light and the wavelength of the continuous light is set to be substantially equal to each other.
According to another embodiment of the present invention, the optical divider is an optical circulator.
According to yet another embodiment of the present invention, the optical wavelength selector is an optical band pass filter.
An optical transmission system according to the present invention, comprises an optical transmitter, an optical receiver, an optical fiber, and an optical amplifier and employs a wavelength multiplexing transmission scheme in which a plurality of signal lights having different wavelengths are composed and transmitted in a single optical transmission path, and characterized by an optical regeneration device on the path of the optical fiber.
This optical regeneration device comprises a wavelength separator, the aforementioned plurality of optical regeneration circuits, and a wavelength divider.
According to a still another embodiment of the present invention, optical amplifiers are provided at an input terminal of the wavelength separator, at input terminals and output terminals of the respective optical regeneration circuits, and at an output terminal of the wavelength multiplexer. Therefore, light intensities of the signal lights to be applied to the wavelength separator, the saturable absorption type optical elements of the respective optical regeneration circuits, the wavelength multiplexer, and the optical fiber can be set independently at respective optimal levels.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.